The process of prilling of a normally solid material by (1) heating the material to above the melting point, (2) spraying the molten material through nozzles into a stream of a cooling fluid, (3) maintaining the resulting solid particles in a fluid bed state until the surface is no longer sticky or tacky, (4) then collecting the solid prills is well known. Prilling of an aqueous solution of paraformaldehyde containing from about 80 to about 90 weight percent formaldehyde into a countercurrent flow of inert gas has been disclosed and claimed in several prior art U.S. patents. The most pertinent of these patents are U.S. Pat. Nos. 3,316,309; 3,595,926; 3,632,655; and 4,036,891.
In these prior art patents, the inventor recognizes the need to cure the solid prills for a time sufficient to provide a dry, non-tacky surface. Thus, in U.S. Pat. No. 3,316,309, the solution of formaldehyde is sprayed into a cooling chamber while passing a current of air having the temperature between 40.degree. and 60.degree. C. upward through the chamber, the inventors claim a non-sticky particulate paraformaldehyde is formed. Later, in U.S. Pat. No. 3,595,926 the same inventor has indicated that while the method is satisfactory on a small scale, the product is not sufficiently free of tackiness and quickly agglomerates causing the process to come to a standstill because of complete clogging of the nozzles. Thus, in U.S. Pat. No. 3,595,926, the inventor claims the critical aspect of the new process is that the temperature of the cooling gas is preferably between -40.degree. C. and +30.degree. C. and the paraformaldehyde prills remain in the fluidized bed of the cooling tower until they have been cooled to a temperature below 40.degree. C., preferably 20.degree. to 30.degree. C. in order to avoid stickiness. The cooled pellets are then dried in a contact dryer to a final formaldehyde content of 92 to 97%.
In U.S. Pat. No. 3,632,655, the inventor sprays a polymerization catalyst onto the surface of the prills as they are being formed in the prilling tower. The catalyst is generally an amine, to provide a dense, highly polymerized, non-tacky surface. The process then calls for curing the prills at a temperature of from about 10.degree. to about 20.degree. C. lower than the softening point of the product for from about 2 to about 50 minutes.
In U.S. Pat. No. 4,036,891, the inventor claims spraying the liquid paraformaldehyde into a chamber in contact with an inert gas, removing the resultant particles at between 40.degree. and 60.degree. C., then passing the particles through a series of at least two other vibrating fluidized beds, fluidized by an inert gas held at below the sticking point of the polymer. It is claimed by the inventor, that it is critical that the temperature of the gas in the second or greater-numbered chamber be higher than the temperature of the gas in the first drying chamber.
In all of the prior art methods, a solid, non-sticky, round particle is formed as described.
In our hands we found that, during storage, the molecular weight of the product as prepared by these methods increased precipitously. Further investigation proved that during storage of a bulk quantity of the prills, a sizable quantity of heat was generated, presumably from the continued polymerization and dehydration of the solid prills. In bags of 2000 lbs or more, temperatures of 75.degree. to 80.degree. C. were encountered within 5 to 8 hours of the filling time and temperatures of at least 75.degree. were still present after 20 hours from filling time. This high temperature in the bulk bags caused the melting point of the paraformaldehyde prills to approach 170.degree. C. within 30 hours of filling.